In dental panoramic and cephalographic radiography it is of utmost importance to ensure by an automatic method the optimal density of the latent image on the image detector, to allow proper detection of the anatomical details and consistency of the diagnostic outcome, and to minimise the need of repeating the x-ray examination so imparting unnecessary dose to the patient.
In dental panoramic radiography, using narrow beam scanning technique, the density is greatly influenced by (1) the individual patient characteristics (age, sex, race and size), particularly on the bone structure where osteoporosis phenomena may be present. Additionally (2) great variations in density occur during the exposure, caused by the different x-ray transmission of the various anatomical regions (temporo-mandibular joint, styloid process, hyoid bone, spine vertebrae, etc.) exposed during the scanning process. Finally (3) various irregularities in the patient denture (amalgam fillings, implants, missing teeth, etc.) may induce false detection and consequently erroneous operation of the automatic exposure control apparatus. While the automatic exposure control apparatus should effectively correct the density variations of the first two kinds, it is desirable that it has a high level of rejection for variations of the third kind (artefacts).
In cephalography, being a stationary radiographic technique, mainly density variations of the first kind are present, in conjunction also with the different patient projections (antero-posterior or latero-lateral).
One important feature is that the automatic exposure control apparatus can operate effectively in the several projection modalities (standard and child projections, transversal slicing, temporo-mandibular joint projections, sinus projections, frontal and orthogonal projections, cephalography antero-posterior and letero-lateral) as provided by the modern dental x-ray panoramic and cephalographic equipments.
Another desirable feature is that the automatic exposure control apparatus is able to compensate for the different sensitivities of the compatible image detectors (radiographic film speed factor, phosphor plate x-ray detectors, etc.).
Prior art automatic exposure apparatus for panoramic x-ray equipment determine imaging parameters (e.g. kV and mA of an x-ray tube, speed of the film drive or of the rotating arm supporting the x-ray tube and the film drive) by measuring the radiation passed through a patient either on the basis of a single sample taken at predefined imaging moment, or by continuous measurement, or by identification and measuring on a selected portion of the jaw (preferably the ramus of the mandibula).
Automatic exposure control by correction of the x-ray tube voltage (kV) is preferable, as it has faster response and provides x-ray energy modulation, with varying penetrating power. Adjustment of the speed of rotating arm or film drive requires huge computational capability, not practical in case of multi-projection equipment. Automatic exposure controls where adjustments are effected on the basis of a single sample are very sensitive to variations in the positioning of the patients and do not compensate for differences in the anatomy of the same. Automatic exposure apparatus with continuous control are very critical, due to the intrinsic difficulty in providing accurate and reproducible dynamic correction in all the anatomical regions, particularly in the spine, without producing asymmetry of the image density or other undesirable effects (vertical bands, artefact shadows, etc.). Identification of particular bone locations is restricted to regions with well defined anatomical structure and cannot be easily extended to varying anatomies as practically needed in multi-projection equipment.
The object of the invention is an automatic exposure control apparatus and method for dental panoramic and cephalographic x-ray equipment, capable of producing optimisation of the grey scale of the latent image in a consistent and reproducible way for all the available projections and regardless of the patient size and anatomy, providing high level of rejection of the artefacts generated by structures inserted into the patient denture.
The invention is founded on the following basic assumptions:
(a) in panoramic projections the optimal controlled parameter is the tube voltage (kV), for its faster dynamic response and varying penetrating power; in cephalographic projections the optimal controlled parameter is the exposure time, which is directly proportional to the output dose.
(b) both in panoramic and cephalographic projections it is preferable that the user selects the patient size, so pre-setting a programmed value of the controlled parameter.
(c) in panoramic projections, due to inherent criticality induced by the individual anatomic differences, the variance in positioning of the patient, the presence of irregular structures in the patient denture, it is wise to support the automatic exposure control operation with a variable kV profile pre-programmed for each available projection.
(d) for increased safety and reliability the automatic exposure control operates in a limited range around the programmed value (and programmed profile) of the controlled parameter.
Herefollowing is a description in greater detail of the invention, based on the exemplary embodiment illustrated in the attached drawings. By the disclosure and the appended claims the features and innovations of the invention will be outlined.